1. Field of the Invention
The present invention relates to a color cathode ray tube, such as is used for a color television receiver or an information processing device, and a method for manufacturing a shadow mask for a color cathode ray tube.
2. Description of the Prior Art
Conventional color cathode ray tubes comprise a glass bulb, a shadow mask, and a mask frame. The glass bulb comprises a panel on which phosphors of several colors are arranged, a funnel connected to the rear side of the panel, and a neck portion formed at the rear side of the funnel. An in-line electron gun for emitting an electron beam is arranged in the neck portion. The shadow mask has a surface with a plurality of apertures corresponding to each phosphor of the panel, which is convex towards the panel side. The mask frame has a wall portion to support a skirt portion of the shadow mask.
In order to block the earth""s magnetic field, an overlapping portion A with which the skirt portion 2 of the shadow mask 1 overlaps a wall portion 4 of the mask frame 3 is made large by enlarging the length of the skirt portion 2 in the direction of the tube axis Z, as shown in FIG. 8.
However, since the length of the skirt portion 2 of the shadow mask 1 in the direction of the tube axis Z in such conventional color cathode ray tubes is large, press forming it causes warps, such as the wrinkles B in the width direction of the skirt portion 2, shown in FIG. 9. Since, after the mask frame 3, to which the skirt portions 2 of the shadow mask 1 have been welded, has been attached to the inside of the glass bulb, the temperature of the glass bulb is rapidly increased to about 400xc2x0 C. in a frit-sealing step, and then decreased from about 400xc2x0 C. to about 100xc2x0 C. in an exhaustion step, processing warps such as the wrinkles B in the skirt portions 2 change, the position of the apertures of the shadow mask 1 that are arranged to correspond with the color phosphors on the panel shifts away from the correct position, and it becomes difficult to ensure correct color rendition because the tolerances for the beam landing positions become insufficient. These problems are even more pronounced when invar is used as the shadow mask material instead of steel.
It is an object of the present invention to solve the problems of the prior art and to provide a color cathode ray tube wherein processing warps such as wrinkles in the skirt portion of the shadow mask are avoided, and correct color rendition can be ensured. It is a further object of the present invention to provide a method for manufacturing a shadow mask for this color cathode ray tube.
A color cathode ray tube in accordance with the present invention comprises (i) a glass bulb comprising a substantially rectangular panel, whereon phosphors of a plurality of colors are arranged, a funnel that is connected to a rear side of the panel, and a neck portion formed at a rear side of the funnel, wherein an in-line electron gun for emitting an electron beam is provided; (ii) a substantially rectangular shadow mask having a plurality of apertures that are arranged in correspondence with the phosphors on the panel; and (iii) a substantially rectangular mask frame having a wall portion that supports a skirt portion of the shadow mask. The surface of the shadow mask with the apertures is convex towards the panel. A central portion of the opposing skirt portion is convex in a direction of the tube axis on the side of the electron gun.
With this configuration, processing warps in the width direction of the convex portions of the skirt portions of the shadow mask can be reduced.
In accordance with the present invention, a method for manufacturing a shadow mask for a color cathode ray tube having a panel uses a first upper die and a first lower die for forming a surface of a flat mask plate having apertures into a convex surface; and a second upper die and a second lower die for sandwiching a peripheral portion of the flat mask plate, which slidably enclose a peripheral surface of said first upper die and said first lower die. The method comprises sandwiching said flat mask plate between said first and second upper dies and said first and second lower dies; displacing said first upper and said first lower die in a vertical direction with respect to said second upper die and said second lower die, whereby the flat mask plate is drawn and a surface of the flat mask plate having apertures is formed into a convex surface towards the panel of the color cathode ray tube; releasing the pressure from said second upper die and said second lower die onto the peripheral portion of the flat mask plate; and forming a peripheral portion of the flat mask plate to be parallel to a tube axis direction of the color cathode ray tube by displacing said first upper and said first lower die even further with respect to said second upper die and said second lower die.
With this configuration, processing warps in the width direction of the skirt portions can be reduced, if for example, the central portions of opposing skirt portions of the shadow mask on the side of the electron gunxe2x80x94corresponding to the peripheral portion of a flat mask platexe2x80x94are formed into convex shapes with respect to the tube axis direction